Semiconductor component manufacturing involves multiple types of processes, which can generally be categorized as front-end processes or back-end processes. Front-end processing includes fabrication of multi-layer semiconductor devices starting from a bare semiconductor wafer to form an array of components or die on the wafer, where each die corresponds to a distinct semiconductor component (e.g., an integrated circuit chip that is intended for incorporation into an integrated circuit package). After front-end processing, the wafer undergoes back-end processing which includes electrical testing of fabricated semiconductor die to determine if the fabricated dies are electrically good or bad; and visual inspection of the fabricated die in accordance with predetermined test criteria.
Process Wafer Map
For each wafer, during an initial front end processing step a “process map” or “processed wafer map” (PW map) is created. The PW map is a digital dataset that provides a record of which of the wafer's die are good, and which are defective, based upon the results of electrical tests and visual inspections performed during front-end and back-end processing.
Typically, wafers are circular. Die at the edges or situated at the outermost borders of a wafer are typically not usable for manufacture. Consequently, a PW map typically stores information for or defining a collective or total interior area or “active area” or “active die area” of the wafer within which processed dies are to be manufactured, where this active area typically spans less than the wafer's total surface area. Die within the active area can be referred to as active die; and die outside of the active area can be referred to unprocessed die, dummy die, or mirror die due to their having a “mirror like” reflective unprocessed surface. In most cases, the PW map contains a dataset principally relating only to die within the wafer's active area, along with some information on dummy or mirror die for purpose of determining and counterchecking the position of one or more reference die. Therefore, the PW Map for a wafer does not represent or completely map to every grid position on the physical wafer, but rather represents a subset of the whole wafer, substantially or essentially corresponding to the wafer's active area in which die 20 have been fabricated. Each wafer carries an associated physical wafer identifier (ID) such as a barcode, and each wafer's PW map is digitally linked to the physical wafer ID. The wafer's PW map “follows” the wafer during front end and back end processing, and is updated on an ongoing basis during each set of electrical tests and visual inspections to which the wafer is subjected. Within the PW map, a data field corresponding to each active area die position is updated with a specific and distinct code that describes in combination whether the corresponding die 20 is electrically good or bad, and whether it is visually good or visually defective.
FIG. 1 illustrates a block diagram organization of particular portions of one representative type of prior art semiconductor component manufacturing system 100 directed to aspects of back-end semiconductor manufacturing processes, as described hereafter.
First Visual Inspection
A first visual inspection system, apparatus, or module 102 is provided, which includes at least one image acquisition system, and which is configured for performing a first visual inspection upon a wafer to identify die on the wafer that are physically defective as a result of not being formed correctly, or having surface defects, or not having incorrect dimensions.
Bumping and Second Visual Inspection
For various types of components, after the first visual inspection, the wafer undergoes a bumping process in which solder bumps are disposed or “bumped” at predetermined positions onto wafer die. A “post-bump” second visual inspection module 104 is configured to visually inspect wafer die to assess the position and planarity of solder balls on the die, and identify those die having solder balls that are out of a predetermined planarity or dimension.
During the first and second visual inspections, should a die be identified as visually acceptable, a visual pass code is registered by the first and/or second visual inspection module 102, 104 within the PW map, in accordance with the die's location on the wafer. For a die that includes a particular type of visual defect, the first and/or second visual inspection modules 102, 104 register a corresponding visual reject code within the PW map.
First Partial Singulation
Following the post-bump second visual inspection, the wafer is mounted on a film frame by way of an adhesive or tacky film, and provided to a first singulation system, apparatus, or module 106, whereupon the wafer undergoes a partial or first singulation that involves physically separating the die on the wafer from each other along x-y gridlines defined thereon such that a physical gap exists between each die, and the die are thus essentially completely electrically isolated from one another. As a result of this partial singulation, the wafer is not completely diced through or singulated, and the dies are still carried by an underlying portion of the wafer.
Electrical Testing
After partial singulation, the wafer is transferred to an electrical testing system, apparatus, or module 108, which carries out a set of electrical tests on wafer die, such as by way of a wafer prober. For each die, a corresponding electrical testing pass or fail code as well as the die's electrical test results are registered within the PW map by the electrical testing module 108.
Final Singulation
After electrical testing, the wafer is transferred to a final or second singulation or dicing system, apparatus, or module 110, and undergoes a final singulation or dicing procedure whereby the wafer is completely diced through and individual die are completely separated or isolated from the wafer and each other. In association with the second singulation procedure, the adhesive film on which the die reside is radially stretched to increase the separation between individual die, to thereby facilitate selective die removal from the film frame during die sort operations, which are further described below. As a result of the second singulation procedure, individual die remain on the film frame in their overall relative positions they occupied on the wafer before and after the first singulation procedure, but with slightly greater inter-die separations. For instance, a wafer manufacturing process may establish on-wafer “streets” between die having a width of approximately 40 microns. After the second singulation procedure, the inter-die spacing on the film frame can be approximately 70-100 microns, depending upon the amount of stretch imparted to the film carrying the die.
FIG. 2A is an illustration of a representative fully diced wafer 5 and a corresponding plurality of singulated die 20 mounted on a film frame 12 by way of a thin adhesive film 11. In FIG. 2A, singulated die 20 are organized in accordance with the grid that defined their layout on the wafer prior to the singulation procedures. As indicated in FIG. 2A and described above, the singulated die 20 are separated from each other by horizontal and vertical dicing grooves or gridlines 30, 32 that correspond to the wafer streets between die 20 along which wafer sawing or dicing occurred. Any given die 20 can be defined to reside at a particular grid position with respect to the gridlines 30, 32. The as-manufactured wafer, and hence the diced wafer 5, can include a reference die 21, as further detailed below.
Die Sorting Operation Cum Third Visual Inspection
After the second singulation or dicing procedure, the diced wafer 5 on the film frame 12 is transferred to a third, subsequent, or final visual inspection module 112, which performs a third visual inspection procedure to identify visual defects imparted to the singulated die 20 as a result of singulation. During the third visual inspection, the PW map is once again updated in with an appropriate type of visual pass/reject code.
Thus, after the final/third visual inspection, the PW map indicates cumulatively the results of all prior electrical tests and visual inspections, indicating which die 20 on the film frame 12 are (a) electrically good/acceptable; (b) electrically bad; (c) visually good/acceptable; and (d) visually defective. With regard to visual defects, the PW map can indicate for each die 20 whether one or more specific types of defects were identified, such as dimensional defects, scratches, chipping, edge non-uniformity, solder bump co-planarity errors, and/or other types of errors, and whether such defects fall within predetermined tolerance criteria.
Based upon electrical testing and visual inspection results in the PW map, a component handler or die sort system, apparatus, or module 114 can perform a die sorting procedure involving the selective removal of die 20 from the film frame by way of a pick and place apparatus or mechanism, where such selective removal is based upon each die's prior electrical testing and visual inspection results. In association with die sort procedure, the PW map is updated to store pick and place encoder positions, values, or counts, which are typically referenced relative to the encoder positions for the reference die 21 (e.g., the center of the reference die 21). Such encoder positions correspond to real or physical space coordinates, i.e., a real/physical space position or location, of each die 20 considered during the die sort operations. The pick and place apparatus includes or is associated with a high resolution imaging system having an image capture device (e.g., a camera). The die sort apparatus 114 includes a film frame expansion table that carries the diced wafer 5 during pick and place operations, in a manner understood by one of ordinary skill in the relevant art.
During die sorting, die 20 that are electrically bad are intended to remain on the film frame 12, such that they will not be incorporated into downstream or final products. Die 20 that are both electrically good and visually acceptable relative to visual defect specifications are intended to be selectively picked from the film frame 12 and transferred to a particular destination, which is typically a tape reel assembly 120, such that they can be used in downstream or final products. In various situations, die 20 that are electrically good, but which may have one or more types of visual defects (e.g., which fall outside of specified tolerance criteria) can be selectively picked from the film frame 12 and transferred to one or more other particular destinations, such as a set of bins or visual reject trays 122, based upon visual defect type, after which such die 20 can be further evaluated or re-worked.
The die sort apparatus 114 selectively sorts die 20 to particular final destinations, such as the tape and reel assembly 120 or a particular reject bin or tray 122, in accordance with a set of selectable or programmably established sort codes that associates particular final die destinations with the PW map codes corresponding to electrical testing and visual inspection results. In a simplest scenario considered hereafter, a simplified set of sort can be defined, which as shown in Table 1 indicates that only electrically good and visually good die 20, i.e., “good die,” are to be picked and transferred to the tape and reel assembly 120 (“PT”); and electrically bad die 20, as well as electrically good yet visually bad die 20, i.e., “bad die,” are not to be picked (“NP”), and hence are to remain on the film frame 12.
TABLE 1Representative Simplified PW Map Code - Sort Code TablePW MapPW MapCodeCodeElectricallyElectricallyBadSort CodeGoodSort CodeVisually Good0NP1PTVisuallyRejectedSymbol/Mark0NP2NPBump Absence0NP3NPBump Planarity0NP4NPDimension0NP5NPScratches0NP6NPChipping0NP7NP
A die 20 that is supposed to remain on the film frame 12 based on preselected criteria can be referred to as a “stay-behind” die. As a result of the selective picking, extraction, or removal of die 20 from the film frame 12 during die sorting, the diced wafer 5 evolves to have a “skeletonized” appearance, becoming a “skeleton wafer.” Thus, prior to die sort operations on the diced wafer 5, the diced wafer 5 is “non-skeletonized,” or is a “non-skeleton wafer”; during die sort operations on the diced wafer 5, the diced wafer 5 becomes progressively skeletonized as more and more die 20 are removed therefrom; and after die sorting operations on the diced wafer 5 are complete or substantially complete, the film frame 12 carries a skeleton wafer.
FIG. 2B is an illustration of a representative skeleton wafer 10 corresponding to the diced wafer 5 of FIG. 2A, from which die 20 have been selectively picked, and on which only die 20 that are (a) electrically bad, and (b) electrically good, yet visually defective, rejected, unacceptable, or unusable die should remain as stay-behind die on the skeleton wafer 10, in accordance with the set of simplified sort codes of Table 1. In FIG. 2B, (a) the presence of electrically defective stay-behind die 24 is indicated by shaded areas, (b) the presence of electrically viable yet visually defective stay-behind die 25 that are supposed to remain on the skeleton wafer 10 is indicated by cross-hatched areas; and (c) the absence of electrically good die 26 from the skeleton wafer 10, i.e., a skeleton wafer die position that is correctly “empty” 26, is indicated by lack of shading or blank areas.
Ideally, after die sorting is complete, (a) all electrically good die will have been correctly removed from the diced wafer 5; and (b) all stay-behind die that remain on the skeleton wafer 10 are die that are supposed to remain on the skeleton wafer 10. Unfortunately, multiple types of errors can occur during die sorting, resulting in stay-behind die being inadvertently picked from the film frame 12; and/or good die which were designated for picking inadvertently remaining on the film frame 12. Such die sorting errors can have significant adverse economic consequences. For instance, if a stay-behind die is incorporated into a downstream or final product, such as a packaged IC or circuit board, the resulting product will be incapable of reliably meeting one or more performance requirements, leading to a likelihood of downstream product failure. At this stage, the economic loss is not just the cost of the manufacturing the bad stay-behind die, but the cumulative cost of manufacturing a defective end product. The testing, recall, and reworking of such end products can result in great losses to all parties involved. Hence it is important to detect errors in die sorting prior to the distribution of tape reels downstream.
Types of Die Sorting Errors and how they Occur
In general, the causes of die sorting errors can be categorized as (A) reference die detection and retraining errors; (B) general die detection failures; (C) die edge translation errors; (D) other translation errors; and (E) other causes of errors, as described in detail hereafter.
(A) Reference Die Detection and Retraining Errors
In order to accurately perform die sort operations, the pick and place apparatus must first correctly identify a reference structure or reference die 21. Referring again to FIGS. 2A, and 2B, the reference die 21 corresponds to a specific wafer location relative to which the position or coordinates of (a) every other die 20 on the wafer, and hence the diced wafer 5, can be referenced or indexed; and (b) die-by-die PW map results can be reference or indexed such that the die sort apparatus 114 can selectively sort the die 20 to intended destinations defined by the sort codes. For instance, in accordance with the simplified sort codes of Table 1, the identification of the position of a reference die 21 is how the pick and place apparatus (a) references the positions of good die and stay-behind die on the diced wafer 5, and thus (b) identifies during die sort operations the die 20 to be picked from the diced wafer 5.
In many situations, the reference die 21 includes or is a unique structural feature on the wafer having a set of distinct patterns that can be automatically recognized by way of a machine vision/image processing algorithm. The unique structural feature can be, for instance, a distinct pattern on a particular die, or a notch or a specific corner of a flat on the outer edge of the wafer. Provided that an algorithm is sufficiently robust, the automatic identification of the reference die 21 is likely to be successful, and die sort operations can be accurately initiated. However, instances exist when the pick and place apparatus is unable to accurately locate or recognize the reference die 21. This can be due to one or more factors, such as:                (i) Lack of accuracy in wafer mounting on the film frame 12 prior to wafer dicing. In this case, the wafer is mounted slightly off of its supposed position by a small or minute amount, e.g., +/−1 mm in vertical and/or horizontal directions. As a result, the camera may not be able to detect the presence of the reference die 21 at an expected location, in which case pick and place operations would stop.        (ii) Stretching of the film. As previously indicated, the film 11 holding the diced wafer 5 is stretched to increase the inter-die separation on the film frame 12 (e.g., from originally about 40 microns to about 70-100 microns) to facilitate pick and place operations. The error in positional displacement for each die 20 is the net increase in inter-die separation resulting from the stretching. This error will be multiplied as the diced wafer 5 is indexed across multiple die positions. Consequently, the stretching of the film 11 on the film frame 12 can cause or contribute to the reference die 21 being indexed slightly off position with respect to the camera, resulting in the pick and place apparatus being unable to detect the presence of the reference die 21. Once again, the pick and place operations would stop.        (iii) The existence of foreign particles on the reference die 21 can cause an imaging system to be unable to detect the presence of the reference die 21 by distorting or changing the characteristic of the edge of the die being detecting, thus adversely affecting the edge detection algorithm(s) used for die detection.        
In any of the above instances, die sort operations will not be able to start as the reference die 21 cannot be detected. User or operator intervention is required to teach or retrain the pick and place apparatus to identify a reference die 21. Through such operator intervention, a wrong reference die 21 can be selected, for instance, a die 20 that is adjacent to an originally intended reference die 21. If the wrong reference die 21 is selected, a serious systemic error arises, which results in the die sort apparatus 114 starting to pick die 20 from the wrong starting point or start die position on the diced wafer 5. As can be expected, bad stay-behind die can be picked in place of good die that are supposed to be picked, resulting in the transfer of bad stay-behind die, which are intended to remain as “NP” or no-pick die on the film frame 12, to the tape reel.
(B) General Die Detection Errors
After a reference die 21 has been identified, the die sort apparatus 114 sequentially displaces or indexes the expansion table to move the diced wafer 5 on a die by die basis (i.e., across an expected inter-die separation distance) and/or a next nearest die by next nearest die basis (when navigating or moving across one to more “empty” grid positions from which die 20 are absent) to ideally position each active area die 20 on the diced wafer 5 beneath the imaging system's camera. At each diced wafer location at which a die 20 is expected to reside, the imaging system attempts to automatically identify the borders, edges, or boundaries of the die 20. If the imaging system can successfully identify the die edges, the imaging system can determine the die's center point, and any die repositioning relative to the center of the camera's field of view (FOV) can occur. The die sort apparatus 114 can subsequently determine whether the die 20 beneath the camera is a good die or a stay-behind die in accordance with the die's location as indicated by the PW map, such that this die 20 can be selectively picked or left on the film frame 12. Based upon the determined die center point, the die sort apparatus 114 can then displace the diced wafer 5 by the inter-die separation distance to a location at which an adjacent die 20 is expected to reside, and so on.
If the imaging system cannot identify the edges of a die 20 that is present on the film frame 12 (e.g., at a grid position at which a die 20 resides, but for which automatic edge detection is not possible), for instance, as a result of foreign particles on the die 20, die sort operations stop. Operator intervention is then required in order to reposition or reindex the diced wafer 5 such that die presence and die detection beneath the imaging system's camera is verified. Such operator repositioning/reindexing of the diced wafer 5 can result in the wrong die 20 being positioned beneath the camera, for instance, a nearest neighbor to the die 20 that should actually be under the camera. As a result, picking operations are re-initiated and continue from the wrong die location, which gives rise to a systemic picking error from that die position onward. This systemic error can result in die 20 being picked to the wrong destination(s) (e.g., stay-behind die being picked and transferred to tape, or good die being picked to a reject bin), and/or good die being left on the skeleton wafer 10.
(C) Die Edge Translation Errors
During die sort operations, the expansion table is indexed to consecutively present each sequential die 20 to the imaging system's camera, such that a selective pick and place operation can take occur at each die location. As indicated above, the imaging system utilizes machine vision/image processing algorithms to recognize die features such as die edges, one or more types of intra-die structures or features, such as a column of solder bumps or circuit lines/metallization can have visual or optical characteristics that emulate the optical characteristics of die edges. FIG. 3 is a representative illustration of a row of die 20 in which each die 20 includes intra-die feature 40, such as one or more columns of solder bumps, which can be misinterpreted by an image processing algorithm as a die edge.
If the image processing algorithm incorrectly identifies a set of intra-die features 40 as a die edge, the image processing algorithm will incorrectly identify the position of the die center, and subsequent expansion table indexing to a next expected die position will include a translation error therein. Depending upon overall die size and intra-die feature positions, such die edge translation errors can amount to a significant fraction of the die's overall span (e.g., approximately 0.3 mm relative to the location of the die's center). Furthermore, by moving or indexing die-by-die across an entire die row, such die edge translation errors can accumulate because the imaging system can continue to incorrectly and unpredictably recognize intra-die features 40 as die edges. Moreover, the stretching of the film 11 on which the die 20 reside can add to and thereby worsen such accumulative die edge translation errors.
As indicated in FIG. 3, this problem normally arises when there are a number of consecutive stay-behind die 20 that are not to be picked within a row of die 20. In such a case, the pick and place apparatus “skips” the stay-behind die 20. In this skipping, the imaging system employs image processing algorithms to identify each die 20 along the row. However, due to the accumulative indexing error described above, the pick and place apparatus can index the diced wafer 5 to the location of the wrong die 20, and hence a stay-behind die can be incorrectly picked to tape.
(D) Other Translation Errors
If navigation or traversal from a current grid position across multiple intervening grid positions to a target grid position is required, a conventional die sort apparatus 114 navigates from the current grid position toward and to the target grid position on a next nearest die by next nearest die basis until reaching the target grid position. The conventional die sort apparatus 114 utilizes this type of navigation technique in order to verify the presence and position of multiple die 20 (e.g., as many die 20 as possible) along a navigation path from the current grid position to the target grid position in order to reduce or minimize the likelihood of accumulative indexing or positional translation errors, which can result in die positioning errors during picking operations. It is to be noted that empty grid positions render the automatic detection of die edges or boundaries within such grid positions for purpose of navigation position verification impossible, and hence traversing across multiple empty grid positions increases the likelihood of accumulative translation errors.
Unfortunately, a conventional die sorting apparatus is not capable of reliable, high accuracy navigation from a current grid position across multiple intervening grid positions directly to a target grid position. Additionally, the aforementioned next nearest die to next nearest die navigation technique is undesirably slow, adversely impacting throughput.
(E) Other Contributions to Die Sorting Errors
Other types of problems can give rise to die sorting errors. For example, the PW map used for die sorting is a local PW map generated from a host PW map that resides on a separate host system. In certain situations, the local PW map can become corrupted relative to the host PW map.
Current Die Sorting Error Detection Techniques
To verify that good die 20 have been correctly extracted from the diced wafer 5, skeleton wafer inspection can occur, either manually or by way of a specific automated procedure, as described in detail hereafter.
Manual Skeleton Wafer Inspection
After die sort operations have been completed for an entire cassette of diced wafers 10 mounted on film frames 12, an operator retrieves a skeleton wafer 10 from the cassette. The operator additionally obtains (e.g., from a back-end manufacturing system) a printout that provides a visual representation corresponding to the PW map. This printout has the same size or diameter as the skeleton wafer 10 itself, and visually indicates for each original die location on the skeleton wafer 10 whether the die 20 at that location should remain on the skeleton wafer 10.
The operator then superimposes the printout over the skeleton wafer 10 under backlit conditions to visually compare the physical skeleton wafer 10 with this printout to manually verify whether electrically bad die had been incorrectly removed from the skeleton wafer 10. Such manual verification is time consuming and prone to errors which adversely affect both throughput and yield. An operator's partial inspection of a single skeleton 10 wafer can require 5-20 minutes or longer, depending upon wafer size, die size, and the operator's manual inspection strategy. More particularly, the printout might not be a perfect match to the skeleton wafer 10, and/or skeleton wafer die 20 can be misaligned relative to the printout. Furthermore, such an operation is simplistic, and the chance of manual operator error can be substantial, particularly in view of technological evolution which has given rise to progressively larger diameter wafers and ever smaller component die sizes. For instance, 300 mm wafers on which 2 mm square or smaller die 20 have been fabricated carries thousands of die 20.
Furthermore, manual inspection takes place only after die sorting has been completed for the entire cassette of skeleton wafers 10. Consequently, if a systemic die picking error occurred that affected multiple diced wafers 5, preventative intervention to avoid the propagation of systemic die extraction errors from one diced wafer 5 to another diced wafer 5 within the cassette is not possible. Hence, large amounts of time, wafer processing resources, and associated manufacturing costs have been wasted.
Automated Skeleton Wafer Inspection
In other present situations, a particular type of automated optical procedure is performed after the die sort apparatus 114 has completed the extraction of die 20 from the diced wafer 5 based upon the wafer's PW map and the sort codes. In such a procedure, a skeleton wafer inspection system uses the same optical inspection system associated with the die sort apparatus 114 during the die sort operations. The skeleton wafer inspection system analyzes the PW map in view of the sort codes. Based upon this analysis, the optical inspection system determines a limited number of PW map zones for comparison against counterpart skeleton wafer zones, for purpose of determining whether die 20 within the skeleton wafer zones under consideration have been properly removed from the skeleton wafer 10. Any given PW map zone corresponds to an array of die 20 on the wafer. The sort codes corresponding to the PW map zone indicate which die 20 within the die array are good die that should have been extracted from the skeleton wafer 10, and which are stay-behind die that should remain on the skeleton wafer 10.
The optical inspection system identifies PW map zones 1, 2, 3, . . . , Z for consideration relative to counterpart skeleton wafer zones based upon predetermined criteria, such as zones having at least a predetermined percentage of no-pick or bad dies, or the largest percentage of bad die that should be present within each zone. For instance, the optical inspection system determines, based upon the data within the PW map and the set of sort codes, the set of PW map zones 1, 2, 3, . . . , Z having the highest count of bad die that should remain on the skeleton wafer 10. These Z PW map zones are selected for comparison with counterpart skeleton wafer zones. The total number of zones Z to be considered, and/or the total number of die D in each zone, can be predetermined or user selectable/programmable for each batch of skeleton wafers.
FIG. 4 illustrates a skeleton wafer 10 mounted on a film frame 12, and Z=5 representative skeleton wafer zones 18 for comparison relative to Z=5 corresponding PW map zones. Each zone corresponds to a 5×5 die array. The skeleton wafer 10 shown in FIG. 4 includes a number of stay-behind die 50, and a number of empty die positions 52; that is, die presence 50 in FIG. 4 is indicated by dark shading, and die absence 52 in FIG. 4 is indicated by lighter shading.
The skeleton wafer inspection system indexes the location of each skeleton wafer zone under consideration relative to the aforementioned reference die 21. More particularly, the optical inspection system determines the locations to which the skeleton wafer 10 should be indexed, relative to the location of the reference die 21, on an expected die position by expected die position or nearest die position by nearest die position basis for determining whether the stay-behind die 50 that reside within each of the Z=5 skeleton wafer zones 18 should actually remain at their respective skeleton wafer positions, and whether empty die positions 52 on the skeleton wafer 10 correspond to die that actually should have been picked in accordance with such die positions within the counterpart Z=5 PW map zones.
Specifically, the optical inspection system (a) selects a first PW map zone for consideration; and (b) displaces the skeleton wafer 10 based upon the skeleton wafer reference die location in accordance with the expected diced wafer inter-die separation distance along x and/or y axes to thereby position a first skeleton wafer zone 18 corresponding to the first PW map zone within the field of view of an image capture device. The optical inspection system then (c) positions a first expected die location within the first skeleton wafer zone 18 beneath the image capture device, and (d) attempts to determine whether a stay-behind 50 remains within this first expected die location, or whether the first expected die location is empty 52. The optical inspection system then (e) determines whether the PW map indicates, in accordance with the sort codes, that a die 20 should remain at or should be absent from the first expected die location; and (f) generates a die sorting error indicator corresponding to the first die location in the event that a die sorting error occurred (e.g., a bad die was inadvertently picked form the first expected die location, or a die 20 incorrectly remains at the first expected die location). The optical inspection system subsequently (g) moves to a next or adjacent expected die location within the first skeleton wafer zone 18 to determine die presence or absence at this die location, and the existence of any die sorting error; and so on for each die location within the first skeleton wafer zone 18.
Following its inspection of each die location within the first skeleton wafer zone 18, the optical inspection system (h) displaces the skeleton wafer 10 based upon the expected diced wafer inter-die separation distance to a second skeleton wafer zone 18, and repeats the foregoing die position by die position optical inspection and die sorting error determination for each die position within the second skeleton wafer zone 18. This procedure continues until the Z skeleton wafer zones 18 under consideration have been inspected relative to counterpart Z PW zones. The skeleton wafer 10 can then be unloaded from the die sort apparatus 114, and a next diced wafer 5 can be considered for selective die picking during die sort operations, followed by the above automated skeleton wafer inspection procedure.
Unfortunately, the foregoing conventional automated skeleton wafer inspection procedure suffers from a number of drawbacks. Firstly, the conventional skeleton wafer inspection uses the same reference die 21 used in the die sort operations, meaning that the skeleton wafer inspection system can suffer from a systemic reference die location error in the same manner as described above. Secondly, the accuracy of conventional automated skeletal wafer inspection depends on the number of wafer zones considered. The greater the number of wafer zones inspected, the more robust and accurate the assessment of whether wrong die 20 have been picked. In an ideal situation, each adjacent skeleton wafer zone 18 across the entire skeleton wafer 10 should be optically inspected relative to its counterpart PW map zone to verify whether all die 20 that were intended to be picked as identified by the PW map and sort codes have been correctly extracted from the skeleton wafer 10, and all bad die identified by the PW map and sort codes remain on the skeleton wafer 10. Unfortunately, inspection of all adjacent skeleton wafer zones 18 relative to the entire pool of adjacently defined PW map zones can take a very significant amount of time, for instance, between 10-20 minutes per skeleton wafer 10 depending upon wafer size and die size, which adversely impacts manufacturing process throughput. Consequently, in order to enhance throughput, only a limited number of zones (e.g., Z=5 die zones) are considered in association with an automated skeleton wafer sampling algorithm. Unfortunately, the inspection of fewer than all skeleton wafer zones 18 relative to their counterpart PW map zones means that die sorting errors can remain undetected. Thirdly, conventional skeleton wafer inspection based on wafer zones requires significant time because it captures images of each and every die individually within each selected zone. As there could be hundreds if not thousands of die 20 within each zone, the more zones selected, the more time is required for skeleton wafer inspection. Lastly, accurately positioning any given skeleton wafer zone 18 and individual die positions therein beneath the image capture device can be difficult because large numbers of die are likely to be absent from the skeleton wafer 10. As a result, indexing the skeleton wafer 10 based upon expected inter-die separation distances, rather than the detection of die edges, can result in skeleton wafer positioning inaccuracies or navigational difficulties, which often leads to inaccurate skeleton wafer inspection results.
A need exists for highly accurate, throughput efficient automated systems and methods of inspecting skeleton wafers 10 to determine whether die sorting errors occurred during the selective removal of die 20 from a diced wafer 5 during die sort operations.